Fix qtenv2.bat to include right mingw path

Task-number: QTBUG-37620
Change-Id: I81f3b5f4bcc23d7ed59fa8d2b3b3bac77578c278
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>